What would you think if Luna came down to the Everfree forest to see what all the hust about then came across Nxy hiding in the bush scare what would her reaction would be?

3475332
I shall call her mini-me.

3475346 lol

3475332
Two options:
1) Quick kill on the abomination.
2) Luna adopts her clone. "Past Sins" then happens on a timeframe of several years due to Nix not having easy access to the Everfree, thus not kick-starting the drama at large.

3475332 Hmm... I think Luna freaks out at first, then decides to take her in to keep an eye on her, although she'd do her best to keep Nyx a secret from everypony. Over time she forms the same motherly bond that Twilight formed with Nyx in the original, though the circumstances lading up to that would be different. Celestia eventually finds out and tries to get Nyx to Spell Nexus, but Luna is not as easily persuaded as Twilight (having your sister banish you to the moon for 1000 years will do that to you). This leads Celestia to fear that Luna wanted to take over Equestria with the help of Nightmare Moon Reborn (or something), and so she battles Luna, and Spell Nexus kidnaps Nyx in the ensuing chaos.

After restoring Nightmare Moon to her former glory, they find that she is... less than happy about the fact that they kidnapped her and want her to become the new ruler. Especially when she finds out that she would have to overthrow- and quite possibly kill- the mare she sees as a mother. She'd probably start by banishing Spell Nexus, then announce that she intends to defeat Celestia anyway, mostly because she doesn't trust her. She eventually does, and is about to banish/kill her when Luna arrives and begs her to stop. After a bit of debate, NMM decided to back down.

Afterwards they try to find out everything they can about NMM's resurrection. Unfortunately, the one pony who knows the most about it is currently taking one giant leap for ponykind. So NMM has to bring him. He doesn't want to talk, but seeing how he's surrounded by three angry alicorns, he has to give in. However, the Nightmare essence within him decides to use the time he uses to explain to gather its strength, and then attacks NMM, attempting to take over so it can become the overlord it always wanted to be. NMM struggles to stay in control, but then Luna calls out to her, reminding her of how much they mean to each other, thus allowing NMM to prevail.

After that NMM releases all her followers, Luna turns NMM back to Nyx, and they all live happily ever after

Now someone go write that one out.

3505390
except Luna names her mini-me instead of Nyx and it's a comedy.

3505570 Hmmm... yes, I think that makes sense

I think anypony would would see Nyx for the first time would freak out...with the except of Pipsqueak.

In seriousness though, if Luna were to try and insta-kill on Nyx, there wouldn't be a potential story. However, Luna would've had to rely on physical strength, because the ritual spell took away the magic in the surrounding area (the reason why Twilight couldn't just Teleport). And not using her magic would mean no insta-kill. So before Luna could harm Nyx, Perhaps she would see that she's not just a Nightmare Moon clone, but is a scared little filly. Like Twilight, Luna would most likely be apprehensive at first, but couldn't in good conscience harm the filly. So Luna would take her back to the castle, if for no other reason than to keep an eye on her to make sure she doesn't try to pull anything (like Twilight).

As for what Luna would do to hide the filly's appearance, I could see one of two things...
1: Luna would disguise Nyx like in the original story, except most likely with a different disguise.
2: Luna could make an excuse as to why she looks so much like Nightmare Moon by making the claim like while Luna was Nightmare Moon, she wanted an heir, just in case Nightmare Moon failed. So she used her own magic and impregnated herself with it. But before Nightmare Moon could teach her "mini-me" the ways of ruling the eternal night, the she was defeated by the elements of harmony. And so technically, the filly was orphaned. And without the guidance of Nightmare Moon, the "mini-moon" had no reason to hate Celestia.
Personally, I think the second option is best suited to start to diverge the story away from the original plot. Celestia would know about who and what Nyx is, though no the real details about how she came into being. Celestia would be apprehensive about Nyx, but trusted Luna well enough to be honest.

It would bother Luna to know that Nyx was afraid of the dark and preferred Celestia's day. Luna would be thankful about this to a certain degree, since it would mean that she would not have a desire to cause an eternal night. But Luna would also feel slightly betrayed that Nyx didn't enjoy the night. Luna has accepted that there are many ponies who have to sleep during the night and can't enjoy it to it's full extent. But Nyx was her own... flesh and blood(?). She was in essence a clone. And realizing how betrayed she felt by this, it actually makes Luna realize that she has grown attached to Nyx. But this creates an inner struggle in Luna. On one hoof, she loves Nyx like a daughter. But on the other hoof, Nyx doesn't love Luna's night.

This is where the story could take a few different paths...
1: The betrayal that Luna feels will cause her to become more distant from Nyx. Nyx, being as sensitive as she is, feels unloved by her surrogate mother. And so, Nyx leaves to travel into the Everfree forest in hopes that somepony else will find her and take care of her. However, she get's lost and is subconsciously led to the castle of the two sisters. There, Nyx recognizes the place and regains some of her memories. And this is where things start to branch off again...
1-A: The magical discharge from the castle catches Luna's attention (as well as Spell Nexus). Luna then goes and generally does what Twilight did in the original story.
1-B: The magical discharge from the castle catches the attention of Twilight (as well as Spell Nexus). Twilight goes to check to see what's going on and find a "mini-moon" in the castle. Twilight would most likely react how she did, or perhaps worse, than when she first found Nyx in the original story. However, she hears "mini-moon" crying and realizes she's just a scared filly. Out of good conscience, Twilight couldn't hurt her. So instead, she takes her back to Ponyville and writes a letter to Celestia. Of course it would be Luna who responds (I'm picturing her response to the letter in Do Princesses Dream of Magical Sheep?). Luna tries to explain that the filly is not a threat. When that doesn't work, Luna claims that she is keeping this filly at the castle to ensure that she doesn't cause trouble. This convinces Twilight to release the filly into Luna's care.
1-3: The magical discharge from the castle catches Zecora's attention (as well as Spell Nexus...as if you need me to tell you that a third time). Zecora then goes through the forest brush, to check on what was all the fuss (I couldn't resist, Zecora and all). Zecora then see's the "mini-moon". At first she's apprehensive. But Zecora being who she is can sense that it was just a scared little filly. This is where things can get interesting, because it's no Zecora taking care of Nyx (in fact, one idea I had was what if Zecora was the one who found Nyx). Personally though, I wouldn't know where to go from here.
1-D: The magical discharge from the castle catches Spell Nexus' attention...just like every other time. You can guess where Spell Nexus finding her would lead.
2: The inner struggle that Luna feels causes Luna to transform into Nightmare Moon. How's that for a plot Twist? And Nyx being "mini-moon" would perhaps absorb some of the excess energy, transforming her into an adult. This could lead to there being a battle between Nightmare Moon (Luna) and Nightmare Moon (Nyx). Seriously, that would be an epic battle. Eventually, Nightmare Nyx would be able to convince Nightmare Luna that she loves her, even if she's afraid of the dark (or something along those lines). Nightmare Luna then reverts back to Luna. However, they aren't sure how to return Nightmare Nyx back. Luna however does, but needs to take a little time to rest before she can. However, before Luna can do so, the Nightmare Moon vs Nightmare Moon battle had gained the attention of several ponies, Spell Nexus being one of them (yes, it's him again). Spell Nexus takes this opportunity to return Nightmare Moon's memories to Nightmare Nyx. Nightmare Nyx banishes Celestia to the sun (or if the season 5 finale has anything to say, the moon), Equestria is cast into eternal night. But Nyx can't bring herself to banish the pony who was her surrogate mother. So Luna take up what would have been Twilight's cell.
I had a third idea, but I can't recall what it was. Maybe it was something to do with Nyx running through Canterlot instead of to the Everfree Forest, since the Everfree Forest would be a long way from Canterlot. I don't know where I'd go from there though.

Because of how that previous situation branched off, I don't think I can continue on. I'll leave the continuations to your imaginations. Plus, this post is long enough as it is. Seriously though, Now I'm inspired to write my own version of how this would go.

3475332 I do think Luna would probably go through some sort of shock phase. She'd want to make sure that this wasn't some trick somepony was playing on her. Maybe using a spell to make some innocent filly look like Nightmare Moon but not actually be her. Luna would probably try to figure out how much Nyx "remembers". But for the most part, it would be a shock. I don't think she would be so callous as to outright kill Nyx. But she might view it as an opportunity to correct a wrong. If such a wrong still needed correcting, that is. After all, Luna has already paid the price for her crimes. I'm not sure if Luna would personally raise Nyx or maybe have someone else do it. Likely someone else, but with Luna having a heavy influence on Nyx's upbringing - similar to how I portray Nyx with Twilight being the mother, but Luna visiting often and acting like an older sister.

3475332

What would you think if Luna came down to the Everfree forest to see what all the hust about then came across Nxy hiding in the bush scare what would her reaction would be?

Luna: (takes one look at Nyx, and yells) DOCTOR!

Doctor: (runs up quickly) What is it Princess Luna?! (sees Nyx) Oh dear. ... DERPY! Get me my sonic screwdriver!

Derpy: (calls out from out of the room) Alright Love Muffin!